The immune system becomes deranged during aging. Autoimmune and neoplastic disease, occurring frequently in old experimental animals and man, may result from immune dysfunction. The proposed program is designed to establish a collaborative center for the study of immune function during aging. The program will unite laboratories at the Sloan-Kettering Institute for Cancer Research and Cornell University Medical College in definitive studies of cell surface determinants, T and B lymphocyte function, and thymopoietin activity during aging in experimental animals, and in normal and progeric humans.